The three-year mapping and monitoring of underground cavity expansion with 2D resistivity survey: What has revealed?

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1 The three-year mapping and monitoring of underground cavity expansion with 2D resistivity survey: What has revealed? Peangta Satarugsa 1, Winit Youngmee 1 and Suvijuk Meesawat 1 1. Department of Geotechnology, Faculty of Technology, Khon Kaen University, Thailand Abstract Dissolution of near-surface salt domes appears to underlie the severe geotechnical and environmental problems in the Northeastern Thailand. Such salt dissolution increases not only a problem of salinity in soil and groundwater but also a problem of underground cavity. Thus this study aimed to apply a 2D resistivity survey for mapping and monitoring cavity expansion. A resistivity profile was conducted in the area nearby the sinkhole surface expression. A survey result indicated that the 2D resistivity survey could be used as a basic tool for identifying the near-surface underground cavity. An anomalous zone of the underground cavity could be distinguished as a low, 0.3 to 1.0 Ohm-meters, resistivity zone surrounded by a higher background resistivity. A cavity image appears as lateral and vertical anomalies. Result from three repeatedly measurement of apparent resistivity data that were acquired at the same month from different years was found to be useful for a determination of underground deformation through time. Keywords: Rock salt, subsurface cavity, resistivity survey Introduction Subsurface geology of the Khorat and Sakon Nakhon basins (Figure 1), covering an area of approximately 50,000 km 2 in Northeastern Thailand, consists of unconsolidated sediments overlaying deformed claystone and rock salt layers (Figure 2). The claystone and rock salt layers were deposited and were later filled with other sediments. The rock salt layers were deeply buried under the younger rock strata and were subjected to a tectonic compressive stress, the rock salt layers behaved as a viscous fluid and they were mobilised and flown upward to form salt domes. The salt domes vary in sizes and shapes with the near-surface depth ranges from meters to 1,200-1,300 meters (Satarugsa et al., 2005). Rock salt is a soluble mineral that is dissolved easily in water. Thus it is envisaged that a collapse of rock salt cavities from the shallow salt domes into sinkholes in the basins becomes one of the predictable geotechnical engineering and geo-environmental hazards in the Northeastern Thailand. Therefore, identification of underground cavity-prone areas can help a loss of lives and reduce a fear of land subsidence. Recently imaging with a 2D resistiviy has been documented to be a powerful method for detectionof cavity/sinkhole (Carpenter et al, 1998; Loke, 1999; Batayneh et al, 2000; Van Schoor, 2002). Under this circumstance, the underground cavity can be distinguished on the basis of resistivity anomaly from background. In this paper we report the results of our attempt to evaluate and to confirm a successful application of the 2D resistivity imaging mapping and monitoring the underground cavity. This study is a part of a research project on sustainable rock-salt exploitation in Northeastern Thailand. Geologic Setting Surface geology of the study area is characterised by sandy clay soil (Figure 2). Underneath the sandy clay soil is claystone and rock salt units (Figure 2). Results from reflection and refraction seismic surveys together with 12 core-drilling wells in thestudy area show depths to rock salt varying from 23 to 145 m. On the top of the rock salt is claystone, with its thickness in a range of 23 to 145 m (Satarugsa et al., 2001; Satarugsa et al., 2004 (a)). Generalized subsurface geologic and resistivity units of the study area are shown in Figure 2. Resistivity contrast property (Figure 2) indicates that the cavity either inside claystone or rock salt and filled

with saline water and pieces of collapsed claystone should be distinguishable on basis of resistivity. Thus, geoelectrical responses were expected to produce changes in resistivity (Figure 2).

2 with saline water and pieces of collapsed claystone should be distinguishable on basis of resistivity. Thus, geoelectrical responses were expected to produce changes in resistivity (Figure 2). Figure 1. A map location of the survey line acquired over an active sinkhole s area. Inset box shows the Khorat and Sakon Nakhon basins. These basins contain an immense volume of rock salt, estimated to be 18 trillion tones (Suwanich, 1986). Figure 2. Schematic geological and resistivity units in the study area where the near-surface salt dome is revealed. Resistivity model shows the cavity in claystone and rock salt. The cavity fills with saline water and pieces of collapsed claystone.

3 Method The Syscal R1 Plus resistivity meter was used for measuring field apparent resistivities along survey lines. The survey lines were measured using Wenner Alpha electrode array (Satarugsa et a., 2004 (b)). Station spacing was 10 meters. The apparent resistivities were measured with electrode spacing of 10, 20, 30,,100 meters. The electrode spacing was kept constant in each measurement but it was progressively increased from one measurement to another until the maximum was reached. The apparent resistivities were inversed into earth models by using a 2D resistivity inversion program, RES2DINV of Loke (1999). A resistivity survey line was collected as line locations shown in Figure 1. It was measured at three different times as shown in Figures 3-5. Figure 3. Resistivity imagaing pseudosection acquired in February 2004 showing measured apparent resistivity data, predicted apparent resistivity data, and inverted resistivity earth model result from the inversion of the measured data. Figure 4. Resistivity imagaing pseudosection acquired in February 2005 showing measured apparent resistivity data, predicted apparent resistivity data, and inverted resistivity earth model result from the inversion of the measured data.

4 Figure 5. Resistivity imagaing pseudosection acquired in February 2006 showing measured apparent resistivity data, predicted apparent resistivity data, and inverted resistivity earth model result from the inversion of the measured data. Results Figure 6 illustrates photographs showing sinkhole expansion together with inverted resistivity earth pseudosections from the survey line that was acquired at the same location at three different times. The apparent resistivity data from the pseudosection in Figure 6a were measured in February 2004 when the sinkhole surface expression had diameter of 15 meters. The apparent resistivity data from the pseudosections in Figure 6b and 6c were measured in February 2005 when the sinkhole diameter was 22 meters and in February 2006 when the sinkhole diameter was 25 meters, respectively. High resistivity at the near-ground surface was interpreted as very low moisture content and low organic matters as exposed at the surface (see picture of the ground surface from a series of photographs in Figures 6 a-c). High resistivities at depth about 50 meters below the ground surface in Figures 6 a-c were interpreted to be results from rock salt unit. By comparing results among Figures 6 a-c, the top surface of rock salt unit appeared to be dissolved and removed. The top surface of the rock salt unit illustrated a blow shape feature (compare among Figure 3-5 or 6 a-c). The blow shape feature appeared to be expanded in vertical and lateral directions through time. This suggested cavity expansion through time. Thus results from Figures 6 a-c suggested that the 2D resistivity survey could be used for mapping and monitoring underground cavity expansion. The zone of underground cavity expansion clearly illustrated corresponding to the surface sinkhole expression. Discussions How the underground cavity developed in claystone: Result from inverted resistivity earth models in Figure 6 c had well-defined the lowest resistivity zone beneath the sinkhole surface expression. The lowest apparent resistivity in the profile was 0.3 Ohm-meter located inside the claystone unit, thus it meant highly fractured claytsone or cavity in claystone. The cavity in claystone could be developed when the rock salt unit underneath the claystone unit was dissolved by groundwater. The groundwater should have had to move in and out from time to time through the rock salt unit. Thus, the cavity was originated in the rock salt unit. As the size of the rock salt cavity was increased, the claystone above the rock salt cavity was gradually collapsed and filled into the rock salt cavity (Figure 2). With this reason, the cavity or highly fracture zone could be developed in the insoluble claystone. Why cavity obviously imaged in claystone unit: Results from Figures 6 a-c obviously display the lowest resistivity anomaly of the cavity in claystone unit instead of the rock salt unit. The cavity occurred absolutely in

both rock salt and claystone units, but the lowest resistivity anomaly is well defined inside the claystone because the resistivity of rock salt is too much higher than the claystone.

5 both rock salt and claystone units, but the lowest resistivity anomaly is well defined inside the claystone because the resistivity of rock salt is too much higher than the claystone. The saline water (20% of salt content in water) has resistivity of 0.05 Ω-meters (Telford et al., 1990) and the wet claystone resistivities in this area range from 1 to 200 Ohm-meters (Satarugsa et al., 2006). Assuming the lowest claystone resistivity was 1 Ohm-meter, the claystone would have to be highly fractured and its fractures would have to be filled with saline water to cause such the lowest resistivity zone. In contrast, resistivities of rock salt measured from core sample are greater than 10 6 Ohm-meters (Satarugsa et al., 2004 (a)). Thus, it is unlikely that the lowest resistivity anomaly could be imaged in the rock salt unit. Figure 6. A series of photographs showing sinkhole expansion together with resistivity imaging pseudosections acquired across the sinkhole. (a) February 2004, (b) February 2005, (c) February 2006.

6 Conclusions Results from this study show that; (1) the low resistivity zone in claystone appears to be associated with highly fractured porous zone filled with saline water, (2) the 2D resistivity survey can detect a highly porous zone filled with saline water near the ground surface, and (3) the 2D resistivity survey can detect an expansion of subsurface cavity if a profile is measured repeatedly at different time. Results of this study indicated that the 2D resistivity imaging could be used for mapping and monitoring underground cavities in the Northeastern Thailand. Although many physical properties influence the apparent resistivities, unconsolidated sandy clay soil, claystone and rock salt could be considered as lateral homogeneous resistivities. The very low, Ohmmeters, resistivity structure surrounded by higher resistivity were suggestive of highly fracture zones filled with saline water. The cavity was originated from the rock salt unit. However, once the cavity in the rock salt unit gradually expanded, the claystone above the rock salt cavity could be collapsed into the rock salt cavity. Thus the cavity was produced in the claystone. Acknowledgements This research was supported by the Research Funds of Khon Kaen University and of the Department of Geotechnology, Faculty of Technology, Khon Kaen University. Comments and editing of this manuscript by Dr Soisungwan Satarug is also gratefully acknowledged. References Batayneh, A. T., Al-Zoubi, and Abdallah, s., 2000, Detection of a solution cavity adjacent to a highway in Southwest Jordan using electrical resistivity methods: J. of Environ. and Eng. Geophys., 5, Carpenter, P. J., Doll, W. E., Kaufmann, R. D., 1998, Geophysical character of buried sinkholes on the Oak Ridge Reservation, Tnenessee: J. of Environ. and Eng. Geophys., 3, Loke, M. H., 1999, Electrical imaging survey environment and engineering studies: a practical guide to 2-D and 3D surveys: Geometrics, San Jose. Satarugsa, P., Thongmee, S., Chaithongsri, P., and Khamcha, C., 2001, Applied geophysical investigation for a rapid detection of sinkhole and rock salt formation in Ban Non Sa Bang, Amphoe Ban Muang, Changwat Sakon Nakorn: J. KKU Res., 6, (in Thai) Satarugsa, P., Youngme, W., Somphadong, S., and Roungrean, L., 2004 (a), Applied geophysical techniques for subsurface mapping and evaluation of natural harzard from a collapse of subsurface cavity in the Northeastern Thailand: Investigative report, Department of Geotechnology, Khon Kaen University, Khon Kaen. (in Thai) Satarugsa, P., Meesawat, N, Manjai, D and Ariwech, 2004 (b), Man-made cavity imaging with 2D resistivity technique: In Reid, S., Wongpornchai, W, and Chantraprasert, S. (eds.), Conference on Applied Geophysics Chiang Mai 2004: Department of Geology, Chiang Mai University, Chiang Mai, Satarugsa, P, Youngme, W., Meesawat, S., 2005, New regional boundary of Maha Sarakham Formation in the Northeastern Thailand: results form 2D seismic mapping. In Wannakao, L., Youngme, W., Srisuk, K., and Lertsirivorakul, R. (eds.), Conference on Geology, Geotechnology and Mineral Resources of Indochina 2005: Department of Geotechnology, Khon Kaen University, Khon Kaen, Satarugsa, P, Youngme, W., Meesawat, S., 2006, The data on resistivities of saline aquifers and subsurface cavity filled with saline groundwater in the Northeastern Thailand: results from in-situ measurements. Submitted to J KKU Res. Suwanich, P., 1986, Potash and rock salt in Thailand. Nonmetallic minerals Bulletin 2: Economic Geology Division, Department of Mineral Resources, Bangkok. Telford, W. M., Geldart, L. P., Sheriff, R. E., and Keys, D. A., 1990, Applied geophysics: 2 nd ed: Cambridge University Press, Cambridge. Van Schoor, M., 2002, Detection of sinkholes using 2D electrical resistivity imaging: J. of Applied Geophys., 50,

Assessment of heterogeneity of an internal structure of an earth-fill embankment with 2-D resistivity survey

Assessment of heterogeneity of an internal structure of an earth-fill embankment with 2-D resistivity survey Peangta Satarugsa, Praiwan Uphatum, Manatchanok Buanark and Sakorn Sangchupoo Department of